1. Field of the Invention
This invention relates to a projection apparatus, and in particular to a projection apparatus for effecting image formation by the use of a light scanning optical system including an electro-mechanical transducer element having a number of minute movable mirrors. As such a projection apparatus, there is a recording apparatus or a display apparatus.
2. Related Background Art
For example, DMD (deformable mirror device) is known as an electro-mechanical transducer element having the minute movable mirrors as described above.
DMD is described in IEE Transaction on Electron Device, Vol. ED-30, No. 5544(1983), and the optical system thereof is disclosed in U.S. Pat. No. 4,454,547.
FIG. 1 of the accompanying drawings is a fragmentary plan view of DMD 7, and reference numeral 3 designates mirrors. When each mirror is in its ON state, the reflected light from the mirror is applied to the surface of a photosensitive medium so that the direction X corresponds to the direction of the rotational axis of the photosensitive medium and the direction Y corresponds to the direction of rotation of the photosensitive medium. That is, the direction X is a direction corresponding to the main scan direction and the direction Y is a direction corresponding to the subsidiary scan direction.
Now, in the conventional DMD, as shown in FIG. 1, two mirror rows have been formed in the direction X with the same spacing as the length of each mirror in the direction X being kept between the mirrors, and the mirrors have been disposed in such a staggered pattern that the two mirror rows do not overlap each other in the direction Y. The two mirror rows have been spaced apart from each other in the direction Y while keeping therebetween the same spacing as the length of the mirror 3 in the direction Y, and subsidiary scanning has been effected at a pitch corresponding to this spacing.
Accordingly, if the DMD and the light scanning optical system are geometro-optically ideal, when all mirrors are in their ON state, subsidiary scanning is effected at a pitch corresponding to the length of the mirror in the direction Y, whereby light dots by the reflected light from the mirrors ought to be formed on the surface of the photosensitive medium without overlapping one another and without any slipping portion. By suitably setting a signal input to a DMD driving circuit, a suitable mirror is caused to be ON-OFF at a suitable time, whereby a desired image ought to be formed on the surface of the photosensitive medium.
In reality, however, there occurs the divergence of reflected light by the curvature of the mirrors of DMD (that is, ideally, only the hinge portions of the mirrors may be curved, but actually the whole of the mirrors 3 is curved) and there also occurs the divergence of light by the aberrations of the other optical system than DMD and further, in DMD, the phenomenon of diffraction occurs because of the mirrors being minute and therefore, the intensity of light of each light dot which the reflected light by the mirrors of DMD forms on the photosensitive medium has such a distribution that in each dot, the central portion is large and the marginal portion is small.
FIGS. 2A and 2B of the accompanying drawings are graphs showing the quantity-of-light distribution in the light dot portion on the surface of the photosensitive medium corresponding to the portion along line x.sub.1 --x.sub.1 of the first row of mirror arrays and the light dot portion corresponding to the portion along line x.sub.2 --x.sub.2 of the second row of mirror arrays when all the mirrors of DMD are in their ON state.
When there are only the aberrations of the other optical system than DMD, there are provided such distributions as indicated by dotted lines in FIGS. 2A and 2B, but in reality, as described above, on the basis of various causes, there are provided such distributions as indicated by solid lines in FIGS. 2A and 2B.
So, if the light dots formed by the first row of mirror arrays and the second row of mirror arrays on the same main scan line on the photosensitive medium at different points of time are combined together, the quantity-of-light distribution thereof will be such as shown in FIG. 2C of the accompanying drawings. When there are only the aberrations by the other optical system than DMD, there is provided a generally uniform quantity of-light distribution as indicated by dotted line, but in reality, there is provided a non-uniform distribution as indicated by solid line.
If the uniformity of this quantity-of-light distribution is aggravated and (I.sub.max -I.sub.min)/(I.sub.max +I.sub.min) exceeds the order of 0.05, irregularity will appear in an image developed by a developing device. Depending on the type of development, such irregularity will appear as a black fringe in the subsidiary scan direction in a white solid image or a white fringe in the subsidiary scan direction in a black solid image.
A similar phenomenon also occurs to the quantity-of-light distribution in the light dot portion corresponding to the portion along line y.sub.1 --y.sub.1 of the mirror 3.
FIG. 3A of the accompanying drawings is a graph showing the quantity-of-light distribution in the light dot portion on the surface of the photosensitive medium corresponding to the portion along line y.sub.1 --y.sub.1 of the mirror when the mirrors of DMD are in their ON state.
When, as described previously, there are only the aberrations of the other optical system than DMD, there is provided such a distribution as indicated by dotted line in FIG. 3A, but in reality, as described above, on the basis of various causes, there is provided such a distribution as indicated by solid line in FIG. 3A.
FIG. 3B of the accompanying drawing shows the state of superposition of the light dot of FIG. 3A in the portion on the surface of the photosensitive medium corresponding to the portion along line y.sub.1 --y.sub.1 of the mirror 3 when subsidiary scanning is effected with the mirrors of DMD being in their ON state.
FIG. 3C of the accompanying drawings shows a quantity-of-light distribution in which the light dot distributions shown in FIG. 3B are combined together. When, as described previously, there are only the aberrations by the other optical system than DMD, there is provided a generally uniform quantity-of-light distribution as indicated by dotted line, but in reality, there is provide a non-uniform distribution as indicated by solid line.
If the uniformity of this quantity-of-light distribution is aggravated and (I.sub.max -I.sub.min)/(I.sub.max +I.sub.min) exceeds the order of 0.05, irregularity will appear in the image developed by the developing device. Depending on the type of development, such irregularity will appear as a black fringe in the main scan direction in a white solid image or a white fringe in the main scan direction in a black solid image.
Thus, a recording apparatus using a light scanning optical system including the DMD as described above has suffered from a problem that print of good quality cannot be obtained.
Such a problem occurs not only to a recording apparatus, but also to other projection apparatus such as a display apparatus. In the case of a display apparatus, a suitable display screen is used instead of the photosensitive medium in the recording apparatus, and subsidiary scanning is generally effected with the display screen being fixed and with the position of the light application from the optical system being moved.